Short essay on Accumulation of Variations

The reproduction of organisms produces variations. The variations produced in organisms during successive generations get accumulated in the organisms. The significance of a variation shows up only if it continues to be inherited by the offspring for several generations.

This will become clear from the following example. Suppose a bacterium produces two bacteria by asexual reproduction. Again suppose that one of the offspring bacterium has a variation due to which it can tolerate a little higher temperature (or little more heat) than the other one.

Now, this variation of little more heat resistance will go on accumulating in the offsprings of successive generations of this bacterium. And this will ultimately give rise to a variant of bacteria which will be highly heat resistant and able to survive even at very high temperatures.

The great advantage of variation to a species is that it increases the chances of its survival in a changing environment. For example, the accumulation of ‘heat resistant’ variation (or trait) in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave or some other reasons. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances and die.

Before we describe Mendel’s experiments for explaining the transmission of characteristics (or traits) from parents to their offsprings or progeny, we should know the meaning of some terms such as chromosome, gene, dominant gene, recessive gene, genotype, phenotype, F_: generation and F₂ generation. These are described on the next page.

Chromosome is a thread-like structure in the nucleus of a cell formed of DNA which carries the genes. Different organisms have different number of chromosomes in their nuclei. A gene is a unit of DNA on a chromosome which governs the synthesis of one protein that controls a specific characteristic (or trait) of an organism.

There are thousands of genes on a chromosome which control various characteristics of an organism. Genes are actually units of heredity which transfer characteristics (or traits) from parents to their offsprings during reproduction. Genes work in pairs.

In diagrams and in explanations of heredity, genes are represented by letters. Genes controlling the same characteristics are given the same letters. For example, the gene for tallness is represented by the letter T whereas the gene for dwarfness is represented by the letter t.

The letters T and t actually represent two forms of the same gene (which controls the length of an organism, say the length of stem of a plant). Please note that genes had not been discovered at the time when Mendel conducted his experiments on pea plants to study the inheritance of characteristics. The term ‘factors’ which were used by Mendel as carriers of heredity information are now known as ‘genes’.

Genes for controlling the same characteristic of an organism can be of two types: dominant or recessive. The gene which decides the appearance of an organism even in the presence of an alternative gene is known as a dominant gene.

It dominates the recessive gene for the same characteristic on the other chromosome of the pair. The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. A single recessive gene cannot decide the appearance of an organism.

The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter. For example, in pea plants, the dominant gene for tallness is T and the recessive gene for dwarfness is t. Thus, when we write the genetic cross for pea plant, then the capital ‘T’ represents ‘tall’ and small ‘t’ represents ‘dwarf’.

Genotype shows the genetic constitution of an organism. In simple words, genotype is the description of genes present in an organism. Genotype is always a pair of letters such as TT, Tt or ft (where T and t are the different forms of the same gene). Thus, the genotype of a tall plant could be TT or Tt whereas that of a dwarf plant is tt.

The characteristic (or trait) which is visible in an organism is called its phenotype. For example, being ‘tall’ or ‘dwarf’ (short) are phenotypes of a plant because these traits can be seen by us or they are visible to us.

The phenotype of an organism is actually its physical characteristic which is determined by its genotype. For example, genotype TT or Tt results in a tall phenotype and the genotype tt results in a dwarf phenotype.

When two parents cross (or breed) to produce progeny (or offsprings), then their progeny is called first filial generation or F1 generation (where F stands for Filial which denotes progeny of a cross). When the first generation progeny cross (or breed) among themselves to produce second generation progeny, then this progeny is called second filial generation or F₂ generation. In other words, the generation produced by crossing two F₂ progeny is called F₂ generation. An example will make it more clear. Mother and father are parental generation. Their children are F1 generation, and the grandchildren are F₂ generation.

Gregor Mendel was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristics from parents to progeny. He did this by using different varieties of pea plants (Pisum sativum) which he grew in his garden.

Some of the characteristics (or traits) of the pea plants whose transmission to progeny was investigated by Mendel were height of pea plant or length of stem of pea plant (tall or dwarf), shape of seeds (round or wrinkled) and colour of seeds (yellow or green). A yet another contrasting characteristics (or traits) investigated were colours of flowers (white or violet).

Mendel chose pea plants for studying inheritance because pea plants had a number of clear cut differences which were easy to tell apart. For example, some pea plants were ‘tall’ (having long stem) whereas others were ‘dwarf (having short stem).

Some pea plants produced round-yellow seeds whereas others produced wrinkled-green seeds, etc. Another reason for choosing pea plants was that they were self pollinating (which enabled them to produce next generation of plants easily). And finally, Mendel chose pea plants to study inheritance (and not animals including human beings) because many generations of pea plants can be produced in a comparatively short time span and their study is much simpler than that of animals.

A new form of plant resulting from a cross (or breeding) of different varieties of plant is known as a hybrid. In monohybrid cross we will study the inheritance generation and second generation progeny.

On the other hand, if we breed two pea plants having contrasting characteristics each (or two traits each) to obtain new plants, then it is called a dihybrid cross. In the dihybrid cross we will study the inheritance of two pairs of contrasting characteristics of p plants such as round-yellow seeds and wrinkled-green seeds.